1. Field of the Invention
The present invention relates to a new and improved wet section of a twin wire papermaking machine.
Generally speaking, the wet section of the twin wire papermaking machine is of the type comprising two revolving endless wires, especially an upper wire and a lower wire, travelling in a predetermined direction of movement. The web-supporting portion of the lower wire has a section which extends in essentially horizontal direction, and at one region the upper and lower wires are guided together. Further, there is provided at least one headbox from which emerges the fiber stock suspension in a wide jet which arrives at the one region between both of the guided together upper and lower wires where there is dewatered the fiber stock suspension and the web formed therefrom.
2. Discussion of the Background and Material Information
As a general rule, such type papermaking machines are employed for the manufacture of a paper or cardboard web --hereinafter sometimes generally simply broadly referred to as a paper web--which is formed by dewatering a fiber stock suspension at a wire or between two wires.
It is well known in the papermaking art that there are available quite a number of different constructions of papermaking machines of the aforementioned type. For example, in the commonly assigned German Patent No. 3,138,133, published Mar. 24, 1983, there is schematically disclosed in FIGS. 1, 2 and 3 papermaking machines which, through the use of an upper wire and a lower wire, render possible the fabrication of a paper 3,910,892, published Oct. 11, 1990, there is disclosed the forming region of a twin wire papermaking machine comprising a forming roll located at the lower wire and a section of the twin wire which extends upwardly over a curved forming shoe.
However, the heretofore known papermaking machines of this type are afflicted with the drawback that the operating range thereof is limited in that such papermaking machines can not be operated at velocities below a predetermined value and then only when producing paper products of relatively low basis weight, for example, paper used for printing newspapers. Such prior art papermaking machines possess the feature that the centrifugal force present at the forming roll, at velocities particularly below 500 meters per minute, is too small in order to adequately upwardly propel the water of the stock suspension and to remove such water at a collecting vat or trough provided for such water. This aspect of water removal is additionally made more difficult due to the fact that such papermaking machines exhibit an ascending course of the forming wire as viewed in the lengthwise direction of the papermaking machine, also referred to as the machine direction, in other words, in the direction of travel of the forming wire. Additionally, by virtue of the ascent of the guidance of the forming wire at the sheet forming zone of the papermaking machine there can arise differential velocities between the fiber stock suspension and the forming wire, resulting in alignment of the fibers in the lengthwise or machine direction of the papermaking machine.
This phenomenon is attributable, on the one hand, to a deceleration of the flow velocity of the fiber stock suspension due to having to overcome an increase in height in accordance with Bernoulli's equation. A further reduction in the velocity of the fiber stock suspension is caused by the presence of additional friction and stock turning losses due to the presence of the suction boxes.
The result of all of this is that there occurs a relatively pronounced fiber alignment in the lengthwise direction of the papermaking machine. The tear length ratio, namely L/Q, measured in the lengthwise and transverse directions of the paper web, can lie in the range of 2.5 to 4, which is frequently undesirable, for example, during the manufacture of liner or test liner paper or board or the like. In that case, there is desired a relatively low L/Q ratio in the order of between 1.0 and 1.5.
Furthermore, when there are present relatively high web weights, drawbacks arise by virtue of the small selected wrap angle at the forming roll of the prior art papermaking machines. At the subsequent region of the papermaking machine there also prevails the danger that the paper web is unduly compressed by the action of the forming elements arranged at the upper wire when there is present a high sheet weight or basis weight of the paper web.
A different construction of papermaking machine, as for example disclosed in U.S. Pat. No. 4,830,709, granted May 16, 1989, is devoid of any forming roll at the lower wire. Moreover, the first suction box at the lower wire, as viewed in the machine direction, is ascendingly arranged. Consequently, the water effluxing through the upper wire can flow back opposite to the machine direction, especially in the presence of relatively low machine velocities. Also, with this construction of papermaking machine there is not present any upper apex point of the lower wire at the front section or region of the sheet formation.